<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>

    <ul>
        <li>1</li>
        <li>2</li>
        <li>3</li>
    </ul>

    <script>

        // 
        var oLis = document.querySelectorAll('li') ;

        for(var i = 0 ; i < oLis.length ; i++) {
            oLis[i].onclick = function() {

                // 把三个都变黑，再把当前这个变红
                for(var j = 0 ; j < oLis.length ; j++) {
                    oLis[j].style.color = 'black' ;
                }

                this.style.color = 'red';
            }
        }
       

    </script>
    
</body>
</html>